This invention relates in general to an electrodeionization (EDI) process wherein liquid to be purified is passed through an ion depletion compartment containing anion and/or cation resin beads under the influence of a polar field to effect ion transfer from the liquid in the ion depletion compartment to a liquid in an ion concentration compartment.
The purification of a liquid by reducing the concentration of the ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of the specific ions or molecules from a liquid mixture. Known processes for purifying liquids include distillation, electrodialysis, reverse osmosis, liquid chromatography, membrane filtration and ion exchange. Another method is electrodeionization.
An early apparatus and method for treating liquids by electrodeionization was disclosed in U.S. Pat. Nos. 2,689,826 and 2,815,320. U.S. Pat. No. 2,689,826, issued to P. Kollsman on Sep. 21, 1954, describes an apparatus and process for the removal of ions within a liquid mixture in a depletion chamber through a series of anionic and cationic diaphragms into a second volume of liquid in a concentration chamber under the influence of an electrical potential which causes the pre-selected ions to travel in a predetermined direction. The volume of the liquid being treated is depleted of ions while the volume of the second liquid becomes enriched with the transferred ions and carries them in concentrated form. U.S. Pat. No. 2,815,320, issued to P. Kollsman on Dec. 3, 1957, describes the use of microporous beads formed of ion exchange resins as a filler material positioned between the anionic or cationic diaphragms. This ionic exchange resin acts as a path for ion transfer and also serves as an increased, conductivity bridge between the membranes for the movement of ions.
The term xe2x80x9celectrodeionizationxe2x80x9d refers to the process wherein an ion exchange material is positioned between anionic and cationic diaphragms. The term xe2x80x9celectrodialysisxe2x80x9d refers to such a process which does not utilize ion exchange resins between the anionic and cationic diaphragms. Illustrative of other prior art attempts to use a combination of electrodialysis and ion exchange materials or resins to purify saline from brackish water are described in U.S. Pat. Nos. 2,794,770; 2,796,395; 2,947,688; 3,384,568; 2,923,674; 3,014,855; and 4,165,273. Attempts to improve electrodeionization apparatus are shown in U.S. Pat. Nos. 3,149,061; 3,291,713; 3,515,664; 3,562,139; 3,993,517; and 4,284,492.
A commercially successful electrodeionization apparatus and process is described in U.S. Pat. No. 4,632,745, issued to A. Giuffrida et al. on Dec. 30, 1986. The apparatus utilizes ion depletion compartments containing an ion exchange solid composition and a concentration compartment which is free of an ion exchange solid material. The electrodeionization apparatus includes two terminal electrode chambers containing an anode and a cathode respectively which are utilized to pass the direct current transversely through the body of the apparatus containing a plurality of ion depletion compartments and ion concentrations compartments. In operation, the dissolved ion salts of the liquid are transferred through the appropriate membranes from the ion depletion compartments to the ion concentration compartments. The ions collected in the ion concentration compartments are removed through discharge outlets and then directed to waste.
In present electrodeionization processes, feed water is initially pretreated in a reverse osmosis step to reduce the ionic load and colloidal contaminants therein, prior to being directed towards electrodeionization. This practice extends the useful life of the resin beads used in electrodeionization. However, even when using a reverse osmosis pretreating step, the presence of certain carbonic species (including dissolved CO2, H2CO3, HCO3xe2x88x92 and CO3xe2x88x922) in the feed water causes problems in the overall process. Generally, ionized carbonic species such as HCO3xe2x88x92 and CO3xe2x88x922 are retained by the reverse osmosis (RO) membrane. However un-ionized species such as CO2 and H2CO3 readily pass through the RO membrane. In electrodeionization, carbonate producing species such as CO2 and H2CO3 can cause so-called xe2x80x9cscalingxe2x80x9d in the ion concentration compartments due to precipitation of calcium ion and magnesium ion at the anionic membrane, particularly at neutral to high pH conditions. Scaling can result in a substantial reduction of the useful life of the electrodeionization apparatus.
Thus, two common problems encountered in the practice of EDI are (a) inadequate ionic removal which can lead to poor water quality and (b) scaling, which when unattended, can quickly lead to premature failure of an EDI module.
A number of factors can lead to poor water quality. However, for a well designed and constructed EDI module, insufficient electrical current is the most common source of poor water quality. This occurs because a certain minimum current is required to remove the ionic contaminants. Furthermore, the higher the ionic content of the feed water, the higher the current required to effectively remove contaminants and produce good water quality. Good water quality is defined herein by the resistivity of the water which is typically desired to be no lower than approximately 3 mega-ohm-cm, more preferably above 5 mega-ohm-cm, and most preferably greater than 10 mega-ohm-cm.
A number of factors can lead to scaling, for example, the presence of hard ions such as Caxe2x88x922 or Mgxe2x88x922 in high concentrations in the water feeding the EDI module. Some manufacturers of EDI modules specify that Caxe2x88x922 levels be maintained below 0.5 ppm to prevent scaling. Scaling typically occurs in the boundary layers adjacent to the cathode and to the anionic membrane on the side facing the waste compartments due to the high pH conditions typical of these regions. In the cathode, electrochemical reactions typically produce hydroxide reactions (OHxe2x88x92); in the waste side of the anion membrane high hydroxide ion concentration occurs as the result of their transport through the membrane. It is believed that the pH at the boundary layer increases with current. Therefore, the current needs to be maintained at a sufficiently low level to prevent or, at least ameliorate, the incidence of scaling.
In view of the above, it is necessary to maintain the current passing through the EDI module within an acceptable range. If the current is too low, poor water quality is obtained. If the current is too high, the incidence of scaling increases.
Presently, EDI modules typically operate using a constant voltage power supply. Unfortunately, it has been observed that the electrical impedance of EDI modules increases with the age of the module. Although the cause of the impedance increase is not known, it is thought to be due to absorption of contaminants into the ion exchange media which, in general tends to increase the specific impedance of ion exchange resins. This impedance increase means that as the EDI module ages, the current passing through the module decreases when powered with a constant voltage power supply. Thus, over time a low enough current may be reached as to result in inadequate water quality. Likewise, a new module having a low impedance and run at constant voltage can produce a very high current thereby increasing the incidence of scaling. Therefore, the aging of the EDI module results in a large variation in current during its lifetime; variations that can produce electrical currents outside a desired operating range.
In addition to reduced longevity, it is also well-known that the impedance of an EDI module increases with decreasing temperature. Thus, during warm summer months, EDI modules may produce very good water quality, while, during the cold winter months when the tap water temperature may be as low was 4 degrees centigrade, the module may not obtain sufficient current to remove all of contaminants when operated under constant voltage conditions. Alternatively, while the risk of scaling may be low in winter, it increases during the summer as the operating electrical current increases. Thus, seasonal variations can also lead to changes in the EDI modules"" electrical impedance, and result in electrical currents outside a desired operating range. Therefore, constant voltage power supplies present a problem if one wishes to maintain reliable performance long-term and across the seasons of the year.
Due to the problems set forth above, undesirable performance, unscheduled maintenance, possible shutdowns, and high operation costs may ensue.
It is an object of this invention to provide a process for the reliable production of good quality deionized water under varying environmental conditions and/or in the presence of undetermined species and concentrations of contaminants in feed water. It is still another object of this invention to provide a robust EDI-based purification process, i.e., one tolerant to extremes in operating conditions encountered in varying potential installations as well as extremes due to seasonal cycles. It is also an object of this invention to provide an EDI module having good longevity.
Accordingly, a need exists for an electrodeionization process configured for a substantially reduced incidence of xe2x80x9cscalingxe2x80x9dxe2x80x94especially with liquid feeds containing substantial amounts of hard ions, such as Ca++ and CO2xe2x80x94and thereby, promoting a substantial increase in the useful life of an apparatus employed for carrying out said process.
The present invention proposes that an optimal current range for operating an EDI module which necessitates the use of a power supply that adjusts the current to maintain it within the desired range rather than utilizing a constant voltage supply. By operating an electrodeionization module within a specific current range, acceptably pure water is obtained while scaling caused by the presence of Catt, Mgtt, and CO2, is substantially reduced or eliminated. When operating the EDI module at a current below the desired current range, the ions are not removed from the feed water thereby preventing obtaining a product having a conductivity within the desired resistivity of 5-15 meg-ohm range. When operating at a current above the desired current range, undesirable scaling within the cells occurs.
The lowest current within the desired current range is determined by mass balance considerations. There is a theoretical minimum current required to remove all ions from a feed water stream. This theoretical minimum is given by a form of Faraday""s Law, derived and based on the recognition that the current inside the EDI module is carried by the ions moving from one compartment to an adjacent compartment. At least one faraday of electric charge is necessary to remove one equivalent of ions from the feed water to the EDI module. Thus, the theoretical minimum current is directly proportional to the ionic load presented to the EDI module. The ionic load is defined as the product of the water flow-rate-per diluting-cell, q[liters/hour] and the ionic concentration of the feed water, C [equiv./liter]. These considerations lead to the following equation:       I    *    =            Q      xc3x97      C      xc3x97      F              3600      xc3x97      N      
wherein,
I*=theoretical minimum current [amps or coulombs/sec.];
Q=product water flow rate to entire EDI module [liter/hour];
C=total ion concentration [equiv./liter];
F=Faraday""s constant=96,500 coulombs/equiv.; and
N=number of cells in the electrodeionization cell.
While in theory a current equal to I* should remove all contaminant ions, in practice, a higher current is required. We have discovered that, to get reliable removal of ions sufficient to produce good water quality, a current at least equal to 1.5xc3x97I* is desired. Furthermore, the maximum operating currentxe2x80x94while greater than 1.5xc3x97I*xe2x80x94should be less than 15xc3x97I*, preferably less than about 10xc3x97I*, and most preferably less than about 5xc3x97I*, in order to minimize scaling.
In the process of this invention, water is introduced into the compartments containing the resin beads while an electrical voltage is applied between an anode and a cathode positioned on either side of the compartments containing the resin beads. The current is monitored and adjusted during processing so that the current is maintained within a predetermined range where good water quality is obtained and scaling in the electrodeionization compartments is minimized or eliminated. Water to be purified is passed through the ion depletion compartments, while water in the concentration compartments, after accepting ions from the ion depletion compartments, is discarded. The electrodeionization step can be operated by passing the water being treated in one pass through a given ion depletion compartment or by effecting serpentine flow within two adjacent ion depletion compartments.
The electrodeionization process described herein is subject to variation. For example, the process can be conducted under conditions where voltage polarity is reversed periodically. Additional process steps can also be added. For example, an ultra-filtration step downstream of the EDI module can further improve product purity or by a preliminary step wherein water to be purified is subjected to reverse osmosis and/or exposed to ultraviolet radiation under a wave length that promotes oxidation of organics, e.g., 185 nm so that substantially complete removal of total organic carbon (TOC) can be effected.
These and other embodiments of the invention, as well as other advantages relating to the practice of the invention, will be better appreciated from the following detailed description construed with consideration of the attached drawings.